(a) Field of the Invention
The present invention relates to a photoreceptor for electrophotography, and more particularly it pertains to a protective film made of amorphous silicon for photoreceptors intended for electrophotography.
(b) Description of the Prior Art
It was reported by Spear in 1976 that amorphous silicon (non-crystalline silicon, hereafter to be called briefly a-Si) which was obtained by processing silane (SiH.sub.4) gas by relying on the plasma CVD (Chemical Vapor Deposition) technique became a useful semiconductor material which permitted the control of the conductivity type and carrier density of the film owing to the fact that those hydrogen atoms which were taken therein were coupled with Si dangling bonds reduced local electron trapping level of the film (Applied Physics Letter, vol. 28, No. 2, January 1976).
Due also to subsequent laboratorial studies which have made it clear that a-Si allows the acquisition of a large-size film at a low cost, a-Si is now becoming a material indispensable for the manufacture of semiconductor devices such as solar battery and thin-film transistor. And, owing to the fact that this a-Si film possesses superior properties such as freedom from pollution, high resistivity and long service life, there has been considered the application of this film to a photoreceptor for electrophotography. However, since those a-Si films which were obtained in their earlier stage of development showed resistance values not high enough for satisfying the requirements of photoreceptors, there has been a delay in putting a-Si films to practical photoreceptors for electrophotography. This delay is considered to be due to the reason that a photoreceptor is of such a physical property that, when a photoreceptor not having a high resistivity is charged with electricity at the surface of the a-Si film by the application of a corona discharge, the photoreceptor exhibits an intensive voltage decay in darkness, leading to a poor charge holdability. In order to compensate for this drawback, consideration was given to the elevation of the resistivity of photoreceptors by, for example, forming a pn junction in the vicinity of the surface layer of the a-Si film by utilizing the controllability of the conductivity type of the layer. This consideration, however, has not been put to practice yet due to various problems and difficulties.
The present inventors attempted to improve the electric charge preservability of the film by elevating the resistivity of the a-Si film per se, and have succeeded in obtaining an a-Si photoreceptor having such a high resistivity as is comparable to that of an Se photoreceptor, and disclosed this success in Japanese Patent Preliminary Publication No. Sho 57-37352. This publication shows the art that an a-Si film is obtained by relying on the CVD technique while mixing an appropriate volume of N.sub.2 gas and B.sub.2 H.sub.6 gas into silane (SiH.sub.4) gas. The a-Si film thus obtained exhibited a markedly high resistivity and also a good photoreceptivity, and in reality an excellent image could be formed. From the practical point of view, however, the above-mentioned a-Si film was not necessarily satisfactory with respect to its service life. The reason therefor was considered to be found in the following. That is, within the apparaus such as a copying machine or a printer, the surface of a photoreceptor is subjected to various stimuli. These stimuli include chemical reactions due to adsorption of ozone and nitrides produced by corona discharge and also due to the deposition of chemically active substances produced by these adsorbed substances and the moisture existing in the air and toners; and also include physical reactions due to abrasion caused by the cleaning plate or due to friction against a paper sheet, and the deposition and diffusion of Na due to finger-touch occurring during the handling operation. These stimuli would more-or-less adversely affect the quality of the image which is to be obtained, and in case these stimuli lasted for an extended period of time, they would bring forth, for example, a marked degradation of the quality of the image as represented by white striations, white spot-like local losses of image, blurring of image and development of fog.
The inventors, therefore, have earlier proposed a method of forming an amorphous silicon nitride (a-Si.sub.x N.sub.1-x) film continuously on an a-Si film as a means of protecting the underlying a-Si film by relying on the same starting gases and using the same apparatus as those employed in the manufacture of a-Si photoreceptor layer, but changing only the operating conditions such as the gas flow rate and the value of the electric power supplied. This proposal is disclosed in Japanese Patent Preliminary Publication No. Sho 58-145951. By the formation of this surface protective layer, the a-Si photoreceptor did reach the stage of practical use with respect to items of both the image formation and the service life. At present, this surface protective layer is not limited to amorphous silicon nitride alone, but study is being made of such films as amorphous silicon oxide and amorphous silicon carbide.
The relationship between the above said surface protective layer and the property of the photoreceptor will be discussed further with respect to the device having a conventional structure by giving reference to FIGS. 1A to 1D. In FIG. 1A, there is formed, on a conductive substrate 1 such as aluminum to a thickness of 1.about.5 .mu.m, an a-Si photoreceptive layer 2 containing hydrogen atoms, made by decomposing SiH.sub.4 gas mixed with N.sub.2 gas, B.sub.2 H.sub.6 gas or PH.sub.3 gas in some cases, by relying on the plasma CVD technique. This a-Si film 2 possesses such a high resistivity as 10.sup.12 .OMEGA..cm or higher. Upon this a-Si photoreceptive layer 2, there is formed continuously to a thickness of 0.01.about.1 .mu.m an insulator layer 3 having a forbidden band width greater than that of the a-Si photoreceptive layer. The energy band structure of this a-Si photoreceptor of FIG. 1A in its equilibrium state prior to actual photographing operation is shown in FIG. 1B. Also, the energy band structure thereof when the surface of the photoreceptor is charged positive by corona discharge is shown in FIG. 1C. Here, symbol E.sub.F represents Fermi level, E.sub.V the top of the valence electron band, and E.sub.C the bottom of the conduction band. FIG. 1D shows the state that carriers are produced when an image light beam has impinged onto the film. Owing to this incident light, there is produced electronpositive hole pairs within the photoreceptive layer, and electrons flow toward the surface side thereof, while positive holes flow toward the substrate 1 side thereof to thereby neutralize the electric charges of the substrate and the surface, respectively. In case there is provided an insulator layer 3 at the surface, electrons will move, and if they pass through the insulator layer 3 due to the tunnel effect and reach the surface, they can neutralize the surface electric charge. However, if the insulator layer is too thick for electrons to cross over the barrier of the insulator layer, they are trapped at the interface between the a-Si layer 2 and the insulator layer 3, and thus the level of the residual potential is determined by the surface charge which has not been neutralized.
In the earlier period of development of art of this field, the insulator layer was considered to be important of its nature as a surface protective layer of photoreceptors. However, in the stage of art wherein it was not possible to obtain an a-Si film having a high resistivity, the role of the insulator layer as a blocking layer was important also for blocking the phenomenon that the surface charge become neutralized by both the travel and the injection of carriers from the photoreceptive layer. For this reason, if the surface insulator layer is formed to a substantial thickness, there will not be developed a tunnel for carriers, resulting in that the residual potential becomes very high, so that there is formed a space charge region in the vicinity of the interface between the photoreceptive layer 2 and the insulator layer 3, which region serving to further block the moving of carriers. Therefore, the thickness of the insulator layer has to be very small such as several tens of .ANG.ngstrom, less than 1000.ANG. at the most. Therefore, when considered from the viewpoint of the surface protecting function, it as not possible to secure a long service life. Conversely, in case the insulator layer is given a sufficient thickness, it is not longer possible to rely on Carlson method, and there is such a problem that one has to consider an altogether different copying system such as the NP method.